HSDPA is a packet-based data service in the 3rd generation W-CDMA (Wideband CDMA) systems, which provides high-speed data transmission (up to 8-10 Mbps over a 5 MHz bandwidth) in CDMA to support multimedia services. This system is evolved from and backward compatible with Release 99 (Rel'99) WCDMA systems.
In order to reach higher peak rate (up to 28 Mbps at Physical layer), MIMO technology is used, in which multiple antennas are implemented at both base stations (Nodes B) and mobile terminals (UE: User Equipment).
MIMO technology is specified by the 3rd Generation Partnership Project (3GPP), which describes MIMO techniques which are considered as 3G mobile standard techniques.
MIMO terminals have to coexist with other ‘legacy’ terminals: i.e. terminals that comply only with earlier releases of the 3GPP standards such as Rel'99 and HSDPA terminals.
MIMO terminals are a special category of the HSDPA terminals, but for simplicity here HSDPA terminals refers to terminals supporting HSDPA but not supporting MIMO technology.
The 3GPP classifies HSDPA mobile terminals into 18 categories according to their data transmission capability, as listed in Table 1 (The TTI refers to the minimum transmission time interval which is allocated to the mobile terminal for receiving data.
The values in the Table 1 indicate HSDPA categories specified in 3GPP Release 7 (additional categories have been specified in Release 8).
TABLE 1Maximumnumber ofbits of an HS-DSCHtransportblockSupportedMaximumreceivedSupportedmodulationsnumber of HSMinimumwithinTotal numbermodulationssimultaneousHS-DSCHDSCH codesinter-TTIan HS-DSCHof softwithout MIMOwith MIMOcategoryreceivedintervalTTIchannel bitsoperationoperationCategory 153729819200QPSK, 16QAM(MIMO notsupported)Category 253729828800(MIMO notsupported)Category 352729828800(MIMO notsupported)Category 452729838400(MIMO notsupported)Category 551729857600(MIMO notsupported)Category 651729867200(MIMO notsupported)Category 710114411115200(MIMO notsupported)Category 810114411134400(MIMO notsupported)Category 915120251172800(MIMO notsupported)Category 1015127952172800(MIMO notsupported)Category 1152363014400QPSK(MIMO notsupported)Category 1251363028800(MIMO notsupported)Category 1315135280259200QPSK,(MIMO not16QAM,supported)Category 141514219225920064QAM(MIMO notsupported)Category 1515123370345600QPSK, 16QAMCategory 1615127952345600Category 1715135280259200QPSK,—16QAM,64QAM23370345600—QPSK, 16QAMCategory 1815142192259200QPSK,—16QAM,64QAM27952345600—QPSK, 16QAM
At the transmitter of MIMO terminals and Nodes B, the information bits are divided into several bit streams and transmitted through different antennas. The transmitted information are recovered from the received signals at multiple receive antennas by using an advanced receiver.
Commonly, in MIMO systems, two parallel data flows at the same transmission power are simultaneously transmitted in the downlink (DL) from two Power Amplifiers (PAs).
The receiver is able to determine which transmitter antenna the received signal comes from, provided different pilots are used per each PA.
There are two ways specified by the 3GPP in order to guarantee that different pilots are used, i.e. that there is “PA diversity”: one is to transmit the Primary Common Pilot Channel (P-CPICH) on one of the two power amplifiers (PA1) and a Diversity P-CPICH on the other one (PA2); another option is to transmit the P-CPICH on PA1 but to send a Secondary Common Pilot Channel (S-CPICH) from PA2.
An efficient usage of Radio Resources when MIMO is activated in the system requires that both PAs utilize the same amount of power even when non-MIMO traffic is present. This is called power amplifier balancing.
In order to ensure PA balancing, MIMO is coupled with the activation of Transmit Diversity modes for all the channels transmitted on a cell (i.e., to be used when transmitting data to existing Rel'99 and legacy HSDPA terminals):                Space time transmit diversity (STTD) utilizes space-time block code (STBC) in order to exploit redundancy in multiply transmitted versions of a signal, that is, the two antennas transmit the same information but each one uses a different coding scheme.        Time-switched transmit diversity (TSTD) uses the information fed back to the transmitter, which decides which one of is antennas is used to transmit each time, and the receiver checks the quality each transmitter antenna (transmitting alternatively) is received with.        Closed-loop feedback transmit diversity (CLTD) applies a weight W by which one of the two antennas is rotated with respect to the other one, so that coherent combination is achieved at the UE antenna input and the same data stream are transmitted on both antennas without any coding.        
The support of the aforementioned Diversity Techniques is specified as mandatory for all user equipment (UE). A dedicated channel being transmitted in any transmit diversity mode may convey the same data, but the transmissions from the two antennas carry a different pilot signal (over a so-called Diversity Common Pilot Channel).
There are other possible approaches to grant power balancing: One solution consists of using an additional carrier (having available one carrier on the first PA and a second carrier on a second PA) paired with a load balancing between carriers. Another way is the Virtual Antenna Mapping described below.
UTRA MIMO Extension 25.876, version 1.80 specifies several transmission mode proposals intended for application with HSDPA, including the so called “MIMO with Virtual Antenna mapping” which adaptively selects the number of antennas from which to transmit as well as selects the best subset of antennas for the selected transmission mode. Virtual Antenna mapping improves the balance of the transmission powers from the two PAs in the low SNR (signal to noise ratio) region. MIMO with Virtual Antenna mapping does not require Diversity CPICH but uses the S-CPICH (Secondary Common Pilot Channel) defined in the UTRAN.
Overall, in the context of this invention, Virtual Antenna mapping refers to any technology implemented before the PAs (usually the baseband) which is able to split the signals intelligently across PAs so that the transmission powers from the PAs are balanced.
In summary, MIMO transmission needs the usage of two PAs and the availability of a diversity pilot (one per each PA), which can be provided by the usage of either a Diversity CPICH (with STTD transmission mode) or a S-CPICH (with Virtual Antenna mapping).
It is also useful to consider the introduction of future technologies in the 3GPP: 3GPP Rel'8 has defined a Dual Carrier feature in which the UE can receive data from two adjacent carriers. The standardization of Dual Carrier requires that transmit diversity is either used in both carriers or in none of them.
Utilization of STTD as a Diversity Technique has been chosen by the 3GPP as the main technique to be used together with MIMO.
However, some trial measurements carried in field by mobile network operators have shown that the STTD activation significantly decreases the performance of some categories of legacy HSDPA terminals already in the market, particularly those ones of category 7 or category 8, when the terminals are operating in both good and medium radio conditions. Performances in good radio conditions are precisely those that allow reaching the highest peak rates offered by the mobile network operators.
The aforementioned problem is linked to the fact that the HSDPA UEs of categories 7 and 8 use a Type 2 receiver (single receiving antenna and equalizer) or a Type 3 receiver (dual receiving×antenna and equalizer), in order to boost the DL peak rate in good radio conditions, but the utilization of STTD with these family or receivers provokes an associated peak rate performance lower than the case in which STTD is not used.
This calls for a solution in order to manage the allocation of the diverse terminals across different and possible configurations.